Various pacemakers are known to the prior art. Typically, these devices consist of a metal casing which houses a pulse generator. A terminal connector, which is in electrical contact with the pulse generator, extends from the casing and provides an interface from which one or more lead wires extend. The ends of these lead wires are connected by tines or helical screws to the tissue to be stimulated (i.e., the epicardial tissue). The casing of the pacemaker is usually coated with a physiologically acceptable resin which insulates the casing both chemically and electrically, from the surrounding tissues.
Pacemakers may be characterized as bipolar or unipolar. In bipolar devices, both poles, or terminals, of the pacemaker are connected directly to the distal tip of the lead wires. The ends of the lead wires are tined or have helical screws and are attached directly to the epicardium. This allows the stimulating pulse current from the device to flow through the active tissues between the anode and cathode.
Other bipolar devices feature concentric lead wires that are insulated from each other. These wires terminate at one end in a stimulator connector and at the other end in a pair of metal terminals that are spaced and insulated from each other. The terminals are inserted through a blood vessel into the interior of the heart where the current pulses flowing between the exposed terminals stimulate the epicardial tissues.
The casing of bipolar and unipolar devices is typically sealed in a protective coating to prevent corrosion of the casing and avoid bio-incompatibility problems that might arise from the material of the casing. The coating further serves to electrically insulate the pacemaker from the surrounding tissue. Without proper electrical insulation, the discharges from the device tend to cause unwanted local muscle stimulation, such as pectoral twitching. Thus, U.S. Pat. No. 3,918,460 (King et al.), U.S. Pat. No. 3,924,640 (King), and U.S. Pat. No. 3,943,937 (King et al.) disclose pacemakers and other implantable medical devices, which are entirely encased in an epoxy resin. U.S. Pat. No. 5,220,929 (Marquit) discloses a covering or "boot" for implantable medical devices such as pacemakers. The covering is a bio-compatible material, such as silicone rubber, which seals the device from fluid intrusion. U.S. Pat. No. 4,010,759 (Boer) teaches the use of a tantalum oxide insulating layer to protect a pacemaker from corrosion. Szyszkowski (U.S. Pat. No. 5,282,841) teaches the use of a conductor ribbon to interconnect implantable devices such as pacemakers. The reference notes that the assembly formed from the conductor ribbon and the implantable devices may be encapsulated within a cast epoxy head.
While the above noted devices may be suitable for particular purposes, these devices are bipolar in nature. However, unipolar stimulation is preferable in many cases. In unipolar stimulation, only one of the output terminals from the pacemaker is connected directly to the heart. A second terminal, called an indifferent electrode, is provided as a large area metal plate which is part of the pacemaker casing. Thus, a return conductive path from the heart to the pacemaker is provided by way of the indifferent electrode.
In many known pacemakers, the indifferent electrode is provided as a window in the insulating material of the casing through which the metallic surface of the casing is exposed. Thus, U.S. Pat. No. 3,735,766 (Bowers et al.) discloses a pacemaker that is encapsulated in a resin of medical grade silicone sealant. A tab is embedded in the sealant that may be removed at the time of implantation to expose a portion of the metal surface of the stimulator. Similarly, U.S. Pat. No. 3,971,388 (Cowdery) discloses a pacemaker that is coated with an elastomeric material so that only a selected region of the housing of the pacemaker is exposed to the body. This region is selected to minimize local muscle stimulation.
When pacemakers first came into use, cardiologists prescribing the devices referred their patients to surgeons, who then implanted the devices. By convention, surgeons approach the patient's chest wall from the patient's right side. Therefore, pacemakers have heretofore been designed for right entry. Thus, the terminal connectors on prior art pacemakers are designed to allow insertion of the lead wires from the left-hand side of the device. The pacemakers illustrated in FIG. 1 of U.S. Pat. No. 3,735,766 (Bowers et al.) and FIG. 8 of U.S. Pat. No. 5,282,841 (Szyszkowski) are illustrative.
Recently, however, cardiologists have begun implanting pacemakers without reference to a surgeon. By their own convention, cardiologists approach the patient's chest from the patient's left side. Unfortunately, when the pacemaker is implanted, the lead wires must be twisted or wrapped around the top or head of the pacemaker. This results in a certain amount of torque in the lead wires. This torque is bothersome to the cardiologist during surgery and poses potential problems to the patient, including displacement of the lead wires, fractures or tears of the insulation, electrical parameter changes due to shorts in the line, muscle irritability at the implant site, and tissue erosion. Any of these events can, in turn, cause a host of cardiac problems, including no output from the pacemaker, pulse rate increases or decreases, capture problems, faster cell depletion, or life-threatening infection.
If the pacemaker is inserted through left entry and is not flipped over, other problems arise. Thus, if the device is situated so that the indifferent electrode is facing inward, it will cause twitching in the pectoral muscles each time the device paces in the unipolar mode. If the device is oriented with the terminal connector on the bottom, the lead wire will necessarily be longer to compensate for the increased distance between the terminal connector and the site at which the terminal end of the lead wire is connected to the epicardial tissue. This increase in the length of the lead wire is undesirable because it increases patient trauma associated with replacement of the pacemaker. After a pacemaker is implanted, the patient's tissues grow over the lead wire, necessitating surgical dissection of the lead wire each time the pacemaker is replaced. Thus, the degree of patient trauma associated with replacement of a pacemaker is affected by the length of the lead wire.
There is, thus, a need for a unipolar pacemaker that may be implanted with equal facility on either side of the chest without causing torque in the lead wires and while still avoiding unwanted local muscle stimulation. These and other objects are achieved by the pacemakers of the present invention, as hereinafter disclosed.